1. Field of the Invention
The present invention relates to conductive paint which is inexpensive and excellent in characteristics.
2. Description of the Prior Art
As is widely known, conductive paint has heretofore been put to various applications in a variety of fields, particularly in the field of electronic appliances and parts. For example, such conductive paint has been used as jumper wires for a printed circuit wiring board, terminals of resistance formed on such printed circuit wiring board, etc.
It is the most important condition that the conductive paint, when it constitutes a part of an electrical circuit such as by printing, spraying, brush-coating, etc., has its specific surface resistance (hereinafter referred to as specific resistance) .rho.' of not higher than 1 .OMEGA./cm.sup.2, preferably of 0.1 .OMEGA./cm.sup.2. As the metals fulfilling this condition, there have been used Au, Ag, etc. generally called noble metals in the form of powder. Ag powder has been mainly used from an economical point of view and, in rare occasions, it has been used in the form of a mixture with powder of Au, Pt, etc.
Since Ag which is a component material of the conductive paint has recently become highly expensive, the cost of the conductive paint containing Ag powder has been increased by several times as much as it used to be. This increase in cost gravely affects the electronic appliances and parts industry. For this reason, various studies and researches have been conducted to date in order to obtain conductive paint by use of metals other than the noble metals in the form of powder. However, no conductive paint which can be substituted for the Ag conductive paint has yet been commercialized.
Conductive paint obtained by using Cu powder as the conductive material and, as the binder, an ordinary thermosetting resins available on the market will be outlined. When the thermosetting resins, is polycondensated to construct, generally giant resin molecules of a three-dimensional network structure, H.sub.2 O is produced as the reaction product, and depending on the kind of thermosetting resin, other by-product are produced, as has widely been known to the art. In this case, a thin layer of copper oxide is formed on the surface of the Cu powder having high activity by the reaction between the by-products and heat. Since Cu powder is readily activated as the particle size thereof becomes smaller, when it is left to stand in the air, it promptly captures oxygen to form a thin layer of oxide on the surface of the Cu powder in most cases. Therefore, at the time of curing of the resin, the volume resistivity of the conductive paint becomes much higher. When metal powder other than the noble metal powder is, therefore, it has been difficult to obtain low resistance paint.
In order to obtain conductive paint having low resistance by use of metal powder other than the noble metal powder, it is deemed necessary to select a thermosetting resin which fulfills the following two conditions.
(I) It does not oxidize the metal powder even when it is stored in the form of paint, but rather has the reducing property. PA1 (II) It absorbs oxygen to cause a polymerization or condensation reaction when paint containing the thermosetting resin is baked at a high temperature.
The condition (I) is required to avoid oxidization of metal powder contained in the conductive paint during its storage. When electrodes, cabie ways, etc. are formed by printing, brush-coating, spraying, etc. using conductive paint and baked at a high temperature, the condition (II) is required to avoid oxidization of the metal powder contained in the conductive paint as coated, and to reduce a thin layer of oxide which may possibly be formed on the surface of the metal powder, so as to constitute a low resistance conductive film or layer.
The invention on the basis of the aforementioned concept is disclosed, for example, in laid-open Japanese Patent Application No. 93394/1976, and No. 115691/1976 both being entitled "Conductive Composition."
However, in these inventions, the specific resistance .rho.' is still high as disclosed in the specification and, therefore, it has not yet come to a point of being used as a substitution for Ag paste.
It has generally been recognized that inclusion of a reducing agent etc. either into metal powder or into a binder for the purpose of preventing the oxidization would deteriorate moisture resistance, heat resistance, etc. of the film formed upon completion of the curing reaction of the paint. Therefore, it is necessary to avoid mixing, filling, etc. of the reducing agent, with a view to preventing oxidation.
The present inventors have conducted studies on the aforementioned conditions (I) and (II) in detail and, as a result, they have found that a further condition should be added to the two conditions in order to make the specific resistance .rho.' lower. That is to say, they have confirmed that the conditions (I) and (II) are necessary conditions, but not sufficient conditions. On the basis of their studies, they have succeeded in obtaining conductive paint usable as a substitution for the known Ag conductive paint.